One of the benefits of this method of continuity inserts for Claremont is that he could come up with an idea in one month, have the Classic X-Men backup establish it as part of a character's continuity and then tie into it in the regular book right away and say, "Oh, you didn't know this was part of this character's back story all along?"
That's precisely what Claremont did in Uncanny X-Men Annual #12, a tie-in with the Evolutionary War that really sort of spoiled the whole "Arthur Adams doing art for X-Men Annuals" thing, as it seems like such a waste to have Adams draw a tie-in to something like the Evolutionary War. Luckily, Claremont tries to do something different with it.
The X-Men travel to the Savage Land, where Terminus has seemingly wiped out much of the populace.
It turns out that, instead, M'Rin has saved the people by bringing them to her dimension and the X-Men follow...
Note that this reveals that Storm has returned to this dimension MULTIPLE times for a number of adventures. It would be interesting to see some of those adventures in the future. Essentially Storm as John Carter of Mars.
In any event, the people of M'rin's dimension help the X-Men take down Terminus...
Terminus turns out to be Garokk (from the previous Savage Land story that was reprinted in Classic X-Men #22) and Storm and M'Rin say goodbye again...
I believe M'Rin has not appeared since, THIRTY YEARS later.
I wonder if Storm still has the cameo?
If anyone else has a suggestion for an interesting plot point that was introduced and then almost instantaneously ignored, drop me a line at firstname.lastname@example.org!